Single-photon light sources are finding increasing use for a variety of applications, including quantum computing and quantum communications. One type of single-photon source utilizes entangled photons, wherein the photons are generated in a manner that results in their “correlation” so that measurements performed on one entangled photon have an effect on the other entangled photon.
One way of generating entangled photons is through spontaneous parametric down conversion (SPDC), whereby an incident photon of a given frequency interacts with a non-linear optical medium to generate two entangled photons whose combined energy and momentum is equal to that of the incident photon. By detecting one of the entangled photons (the “signal” photon), then one is assured that its partner (the “idler” photon) is present.
Polarization entanglement between 702-nm photons produced via SPDC of 351-nm pump light in a pair of bulk BBO crystals has been recently demonstrated. While this approach works well for proof of principle experiments, beam walk off and a short interaction length tend to limit the conversion efficiency, making bulk crystals unsuitable for building a compact low-power-consumption photon source.
As has also recently been shown, surface waveguides embedded in periodically poled bulk substrates can exhibit a much higher conversion efficiency and require lower pump power in order to achieve the same output photon flux. The high efficiency and low power consumption of the waveguides makes them a superior choice when weight carrying and power production capabilities are restricted, such as for example, space exploration spacecraft or most commercial applications that might use a single-photon light source, such a quantum key distribution systems.
While parametric downconversion has been investigated in great detail in many different bulk crystals, there appear to be only two suitable waveguide candidates: periodically poled LiNbO3 (PPLN) and periodically poled KTP (PPKTP). While PPLN might have stronger non-linearity and, consequently, higher conversion efficiency, it is susceptible to producing a higher proportion of non-correlated photons that can pollute the communication channel of a quantum communication system.